The invention relates to a sheet feeder comprising a suction head which is arranged over a rising table for a stack of sheets and bears at least one sucker able to be connected with a source of vacuum and with air synchronously in steps with the operation of the equipment, a cam predetermining such steps and, following the suction head, sheet conveying means.
In the case of conventional sheet feeders of this type the connection of the suckers with the vacuum and the supply of air thereto is timed by valves arranged in the suction head. These valves are generally in the form of plate or rotary piston valves which have a large flow orifice. Apart from design complexity in conjunction with the incorporation of such valves in the suction head, there is the further disadvantage that owing to the orifice section and the large number of bends there will be substantial reductions in the flow rate and in pressure. More particularly, there is the disadvantage however that the vacuum paths between the vacuum source and the means operated thereby, that is to say between the sucker and the valve arranged in the suction head are very long. The establishment and release of the vacuum should take place as smartly as possible in this case, since long ducts have to be emptied and filled with air. The consequence of this is that a completely regular sheet transfer to the further parts of the conveying system in the form of intermittently operated rolls synchronized with the sheets, or similar conveying means, is practically not possible. In fact there is the danger that even on transfer from the said conveying means the sheet will be gripped before it has been released by the suckers and vice versa. A further difficulty which necessarily occurs is that the point in time of the arrival of the sheets at the front guides of a following machine processing the products will vary, this being likely to interfere with proper operation of the plant. Furthermore, rough handling of the sheets will be likely.